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Configure client-specific message size limits

01/26/2017

7 minutes to read

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In this article

Applies to: Exchange Server 2013

In Microsoft Exchange Server 2013, there are several different message size limits that apply to messages as they travel through your Exchange organization. For more information, see Message size limits.

However, there are client-specific message size limits you can configure for Outlook Web App and email clients that use ActiveSync or Exchange Web Services (EWS). If you change the Exchange organization-wide message size limits, you need to verify that the message size limits for Outlook Web App, ActiveSync, and Exchange Web Services are set accordingly. You configure these values in web.config files on Client Access servers and Mailbox servers. These limits are described in the following tables.

By default, there is no maxAllowedContentLength key in the web.config files for ActiveSync. However, the maximum message size for ActiveSync is affected by the maxAllowedContentLength value that is applied to all web sites on the server. The default value is 30000000 bytes (30 MB). To see these values for ActiveSync on Client Access Servers and Mailbox servers in IIS Manager, perform the following steps:

On Mailbox servers, open IIS Manager, navigate to Sites > Exchange Back End and select Microsoft-Server-ActiveSync.

Verify Features View is selected, and double-click Configuration Editor in the Management section.

Click the dropdown arrow in the Section field, navigate to system.webServer > security and select requestFiltering.

In the results, expand requestLimits, and you'll see maxAllowedContentLength and the default value 30000000 (bytes).

To change the maxAllowedContentLength value, enter a new value in bytes, and click Apply. You need to change the value on Client Access servers and on Mailbox servers. After you change the value in IIS Manager, a new maxAllowedContentLength key is written to the corresponding web.config file (%ExchangeInstallPath%FrontEnd\HttpProxy\Sync\web.config on Client Access servers, and %ExchangeInstallPath%ClientAccess\Sync\web.config on Mailbox servers).

To change the maximum message size for ActiveSync clients, you need to change the value of maxRequestLength in the web.config file on Client Access servers and Mailbox servers, MaxDocumentDataSize in the web.config file on Mailbox servers, and maxAllowedContentLength in IIS Manager on Client Access servers and Mailbox servers.

Exchange Web Services

Serve role

Configuration file

Keys and default values

Size

Client Access

%ExchangeInstallPath%FrontEnd\HttpProxy\ews\web.config

maxAllowedContentLength="67108864"

bytes

Mailbox

%ExchangeInstallPath%ClientAccess\exchweb\ews\web.config

maxAllowedContentLength="67108864"

bytes

Mailbox

%ExchangeInstallPath%ClientAccess\exchweb\ews\web.config

14 instances of maxReceivedMessageSize="67108864"

bytes

Comments on Exchange Web Services limits

There are 14 separate instances of the value maxReceivedMessageSize="67108864" that correspond to different combinations of bindings (http and https) and authentication methods.

To change the maximum message size for Exchange Web Services clients, you need to change the value of maxAllowedContentLength in both web.config files, and all 14 instances of maxReceivedMessageSize="67108864" in the web.config file on Mailbox servers.

In the web.config file on Mailbox servers, there are also two instances of the value maxReceivedMessageSize="1048576" for UMLegacyMessageEncoderSoap11Element bindings that you don't need to modify.

maxRequestLength is an ASP.NET setting that's present in both web.config files, but is not used by Exchange Web Services, so you don't need to modify it.

Outlook Web App

Server role

Configuration file

Keys and default values

Size

Client Access

%ExchangeInstallPath%FrontEnd\HttpProxy\owa\web.config

maxAllowedContentLength="35000000"

bytes

Client Access

%ExchangeInstallPath%FrontEnd\HttpProxy\owa\web.config

maxRequestLength="35000"

kilobytes

Mailbox

%ExchangeInstallPath%ClientAccess\Owa\web.config

maxAllowedContentLength="35000000"

bytes

Mailbox

%ExchangeInstallPath%ClientAccess\Owa\web.config

maxRequestLength="35000"

kilobytes

Mailbox

%ExchangeInstallPath%ClientAccess\Owa\web.config

2 instances of maxReceivedMessageSize="35000000"

bytes

Mailbox

%ExchangeInstallPath%ClientAccess\Owa\web.config

2 instances of maxStringContentLength="35000000"

bytes

Comments on Outlook Web App limits

In the web.config file on Mailbox servers, there are two separate instances of the values maxReceivedMessageSize="35000000" and maxStringContentLength="35000000" that correspond to http and https bindings.

To change the maximum message size for Outlook Web App clients, you need to change all of these values in both files, including both instances of maxReceivedMessageSize and maxStringContentLength in the web.config file on Mailbox servers.

In the web.config file on Mailbox servers, there is also an instance of the value maxStringContentLength="102400" for the MsOnlineShellService binding that you don’t need to modify.

For all message size limits, you need to set values that are larger than the actual sizes you want enforced. This increase in values is necessary to account for the inevitable message size increase that occurs after the message attachments and any other binary data are Base64 encoded. Base64 encoding increases the size of the message by approximately 33%, so the values you specify for any message size limits are approximately 33% larger than the actual usable message sizes. For example, if you specify a maximum message size value of 64 MB, you can expect a realistic maximum message size value of approximately 48 MB.

What do you need to know before you begin?

Estimated time to complete: 15 minutes

Exchange permissions don't apply to the procedures in this topic. These procedures are performed in the operating system of the Exchange Server.

Changes you save to the Web.config configuration file are applied after you restart IIS.

To allow for a 33% increase in size due to Base64 encoding, multiply your desired new maximum size value in megabytes by 4/3. To convert the value into kilobytes, multiply by 1024. To convert the value into bytes, multiply by 1048756 (1024*1024). Note that the size increase caused by Base64 encoding could be greater than 33%, and depends on several factors, for example, the attachment file size, type, compression, and the email client used to compose and send the message.

Any customized per-server settings you make in Exchange XML application configuration files, for example, web.config files on Client Access servers or the EdgeTransport.exe.config file on Mailbox servers, will be overwritten when you install an Exchange Cumulative Update (CU). Make sure that you save this information so you can easily re-configure your server after the install. You must re-configure these settings after you install an Exchange CU.

Find the relevant keys in the appropriate web.config files as described in the tables earlier in the topic. For example, for Exchange Web Services clients, find the maxAllowedContentLength key in both files and all 14 instances of the value maxReceivedMessageSize="67108864" in the web.config file on Mailbox servers.

Configure client-specific message size limits from the command line

Instead of using Notepad, you can also configure the client-specific message size limits from the command line. Open an elevated command prompt on the Exchange server (a Command Prompt window you open by selecting Run as administrator) and run the appropriate commands for the limits that you want to configure.

Notes:

The size values in the commands are the default values, so you'll need to change them.

How do you know this worked?

To verify that you have successfully configured the client-specific message size limit, you need to send a test message to and from a mailbox that's being accessed by the affected client. You can try a few smaller attachments or one large attachment so the test messages are approximately 33% less than the value you configured. For example, a configured value of 85 MB results in a realistic maximum message size of approximately 64 MB.

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